This invention relates to a system for the control of temperatures in the bore of a plastics extruder, and more particularly to a system that controls bore temperature on the basis of two temperature sensors in each of a series of temperature control zones, one sensor being located in a deep well proximate the bore, and the remaining sensor being located in a shallow well near the surface of the barrel of the extruder.
In plastics extrusion lines, plastic pellets are introduced into the bore within an extruder barrel from a hopper. A screw of varying pitch moves the pellets towards the remote end of the bore where an extrusion die forms the extrudate or melt into sheet, hollow tube, pipe, hose, rod, or other arbitrary shapes suitable for such items as, for example, window molding. Typically, four to six different temperature zones are arranged longitudinally along the extruder barrel from the input hopper to the extrusion die. The pellets are melted as they move in the bore. The screw pitch varies along its length in accordance with the function it serves, first moving the pellets as they melt, then mixing the plastic that has melted, and finally, forcing the plastic melt through the die.
Commonly, each of the four to six temperature zones is maintained at a different temperature, to accomplish different objectives. The zone nearest the hopper will merely convey the solid plastic, melting it as it goes along. The next zone might be a mixing zone, with very high pressure and shear, which mixes the plastic thoroughly. This zone will often have a very different temperature from the first zone, as will the temperature zones remaining before the plastic is forced through the die. Heat is supplied to each zone by a zone heater which characteristically is a large aluminum casting with a calrod element in it. These heaters clamp onto the outside of the barrel.
Considerable frictional heat is generated by the screw as it turns in the plastic. An extruder screw may be driven by a 150 to 800 Hp motor, and much or almost all of that energy may ultimately be turned into heat. At high production rates, the heat generated exceeds the amount necessary to heat the plastic to the melting point, melt it, and then raise the melt to the desired temperature. In this case, in order to hold the temperature to the desired point, the temperature controller must supply cooling, as well. This is done by controlling the flow of a coolant through channels in the clamped-on heaters. It will be understood that although the term "heaters" is used herein, these are really temperature altering means, in many cases providing cooling or heating, as required.
Obviously, the temperature which is of most interest is the temperature of the plastic in the barrel. However, this cannot be controlled directly. The next best thing to do is to control the temperature of the barrel, very near the plastic. The straightforward way to do this is to drill a hole down into the barrel as close to the plastic as is mechanically feasible, and then to put a temperature sensor into this deep well. Until 1967, though, this was not feasible. The time lag between a heater on the outside of the barrel and the sensor deep inside the barrel was too great. The controller that energized the heater in response to sensed temperature would go into oscillation.
To see why this happens, assume a time lag of six minutes, which is not atypical, from the moment an increment of heat is added at the heater on the outside of the barrel to the time that heat is sensed at the sensor deep in the well. If the momentary temperature at the sensor is too low, the controller will add more heat at the heater. But it will be six minutes before the sensor recognizes that heat has been added, and by the time that the sensor gets the message, far too much heat has been added. The temperature overshoots. The sensor, of course, detects the temperature overshoot, it signals the controller, and the controller and the heater slams full into cooling. Again it is six minutes before the first cooling effect is felt at the sensor, and again there has been too much cooling, another correction is attempted and so the system oscillates.
In 1967 a new system introduced the concept of automatic reset in extruder controllers. As described in the inventor's article entitled Fundamental Analysis of Extruder Temperature Control, Modern Plastics, August 1967, McGraw-Hill Inc., New York, N.Y., automatic reset introduced an averaging function into the control loop, the loop that included the sensor, the controller, and the heater. Instead of responding to the instantaneous temperature, the automatic reset controller responded to the average temperature over a period of time. In the above example, if the averaging, or reset time is larger than six minutes, the controller would not oscillate as described, but would remain steady at the value necessary to hold temperature at the set point.
In about 1973, a system was advertised that used two sensors, one in a shallow well, and the other in a deep well. This system, it was said, responded faster to external changes.
There is considerable temperature gradient between the inside and the outside of any extruder barrel. This gradient can be as much as 100.degree. F. (56.degree. C.). Moreover, this gradient will be directly dependent on the amount of heat flowing; that is, it will change. It is easy to see, then, that weighting equally a sensor in a shallow well and another in a deep well is the equivalent, as far as the steady state conditions are concerned, of a single sensor midway between them. If the midpoint is held at the set point temperature, which is the temperature set into the controller as the desired temperature, then the deep well temperature, that which is closest to the plastic in the bore, will vary from the desired or set point temperature. In other words, the system introduced in 1973, with a shallow and a deep sensor, did indeed respond faster, but it controlled the wrong temperature.
In one system of the kind just described, temperature indications of the two sensors, the shallow and deep sensors, are simply averaged, and it is this average that the controller uses to control the supply of heating and cooling. In another system of this kind, the temperature indication of the deep sensor is weighted more heavily than that of the shallow sensor, because of the deep sensor's proximity to the plastic in the bore. This second alternative recognizes that it is the deeper sensor that is more significant, but it does not solve the problem. The temperature that is used to control the heater is still the wrong temperature. Weighting the temperature indication of the deep sensor only locates the controlled temperature at a location closer to the deep sensor than to the shallow sensor.
To control the actual temperature of the melt at the extrusion die downstream of the screw, it is known to place a temperature sensor at this location and to use it to control the set points of the controllers that control the heaters in the upstream zones. This technique controls the very important melt temperature near the point of extrusion, but it does so only by modifying the temperatures at all of the temperature zones upstream. This does not provide for deep well temperature sensing in a single zone to correct the temperature there independently. If, for example, the temperature drops in a zone where mixing of the plastic occurs, and this temperature drop results in a lower melt temperature near the die, then adjusting upwardly all of the set points may raise the melt temperature near the die, but it does not correct the error where it occurs, and it is not certain to result in the correct temperature for mixing where the erroneous temperature occurred.